The present invention concerns the field of active transponders and, more precisely, active transponders able to be switched into passive transponders.
There conventionally exist a large number.of portable devices for the contactless identification of an unknown object. A portable device of this type contains data able to be transferred, in the form of radioelectric signals, between a fixed base station acting as transceiver and this device. A portable device of this type is usually made using a transponder.
One will recall that there conventionally exist two types of transponders: passive transponders and active transponders.
A passive transponder is arranged for converting a radioelectric signal originating from a base station, into an electric voltage which then supplies the electric power necessary for the transponder""s operation.
A passive transponder of this type includes an antenna to be able to receive and transmit radioelectric signals containing data, a storage capacitor connected to the antenna, to be able to store the electric power received by the antenna, and an integrated circuit connected to the antenna and to the storage capacitor, to be able to process the data received by the antenna, and to provide thereto other data to be transmitted in the form of radioelectric signals. It will be noted that, in such a transponder, the storage capacitor supplies electric power to the integrated circuit.
One drawback of such a passive transponder is that contactless interrogation of the transponder requires a small distance between the base station base and the transponder, this distance being of the order of several meters. It is necessary for the passive transponder to be at a small distance from the base station for the transponder to receive sufficient power for its operation. In the event that such a passive transponder is incorporated in a vehicle door key, the user of the key must therefore be a short distance from the vehicle.
An active transponder is arranged for transmitting data over long distances of the order of several hundreds of meters. Indeed, contactless interrogating such an active transponder does not require a short distance between the base station and the transponder, since no transfer of energy occurs from the base station to the transponder.
FIG. 1 shows an active transponder of this type designated by the reference 1. This transponder includes an antenna 3 arranged so as to be able to receive and transmit radioelectric signals 4 containing data, an integrated circuit 5 connected to antenna 3, to be able to process data received by antenna 3, and to provide the latter with other data to be transmitted in the form of radioelectric signals. Active transponder 1 further includes a battery 7 able to supply power to the various components of transponder 1.
One drawback of active transponder 1 is that it operates normally, as long as battery 7 provides sufficient electric voltage to supply the set of components of transponder 1. In the event that the latter is incorporated in a vehicle door key, as soon as battery 7 no longer supplies a sufficient voltage level to assure the proper operation of transponder 1, the latter no longer allows the door opening command to be provided, which is generally inconvenient for the user of the vehicle.
There also exists in the state of the art  less than  less than mixed greater than  greater than  or  less than  less than dual greater than  greater than  active transponders, i.e. active transponders which can be switched into passive transponders. GB Pat. No. 2,292,866 discloses such a dual transponder.
FIG. 2 of the present description shows a dual transponder 10 which includes an antenna circuit 11, a modulation-demodulation circuit 14, a rectifier circuit 15, a storage capacitor 16, a battery 18, a central processing unit 20, a read only (ROM) memory 21, a random-access (RAM) memory 22, a clock system 23 and a switching circuit 26.
Antenna circuit 11 is arranged for receiving and transmitting data to and from an external device in the form of a radiofrequency wave 12. For this purpose, antenna circuit 11 includes an antenna 11a formed by a coil and a capacitor 11b.
Modulation-demodulation circuit 14 is arranged for modulating a digital signal received from a control circuit 25 into an analogue signal, and for demodulating an analogue signal received via antenna 11a into a digital signal. The reference 13 designates a line via which a received signal is transferred from antenna 11a to demodulation-modulation circuit 14, and via which a signal to be transmitted is transferred from modulation-demodulation circuit 3 to antenna 11a. Rectifier circuit 15 is formed by a bridge circuit comprising diodes 15a to 15d for converting an alternating voltage from radiofrequency wave 12 received by antenna 11a into a DC voltage V2. Storage capacitor 16 is arranged for storing rectified DC voltage V2, this latter being intended to be provided to the different components via a connection line 17.
Central processing unit 20 is arranged for processing data in accordance with an execution programme. ROM memory 21 is arranged for storing the execution programme. RAM memory 22 is arranged for temporarily storing data. Clock system 23 is arranged for providing a clock signal in response to which processing unit 20 operates. The reference 24 designates a bus via which data and addresses are transmitted.
Battery 18 is arranged for providing an electric voltage V1 intended to supply the various components of transponder 10, via a connection line 19. It will be noted that the operating voltage of transponder 10 is of the order of 5 volts.
Switching circuit 26 is connected to lines 19 and 17, and is arranged so that, when voltage V2 across the terminals of storage capacitor 16 is greater than voltage V1, transponder 10 operates by using the electric power supplied by storage capacitor 16, via line 17, and so that, when voltage V2 is less than voltage V1, transponder 10 operates by using the electric power supplied by battery 1, via line 19.
One drawback of transponder 10 lies in the fact that, once battery 18 has run down, the electric power supply of the components of transponder 10 can only be supplied by storage capacitor 16. In the event that transponder 10 is incorporated in a vehicle door key, the transponder must be situated at a short distance from the base station so as to receive sufficient power from the base station to operate. In other words, once battery 18 has run down, the user of the key has to wait the time necessary for capacitor 16 to charge, before identification can occur, which can be particularly inconvenient for the user.
An object of the present invention is to provide a dual transponder which overcomes the drawbacks of conventional active transponders, in particular an active transponder which allows a power supply level sufficient to assure its operation to be maintained.
Another object of the present invention is to provide an active transponder which answers the conventional criteria in the semiconductor industry as to complexity and cost.
These objects, in addition to others, are achieved by the active transponder according to claim 1.
The invention concerns an active transponder including in particular first and second means for comparing respectively the first and second supply signals to a minimum threshold provided by first supply means and, in response, to supply respectively first and second control signals and charging means controlled by the first and second control signals, and arranged so that, when the first and second supply signals are respectively less than and greater than the minimum threshold, the accumulator is charged, via the charging means, from the energy contained in the storage means.
One advantage of such an arrangement of the charging means and the first and second comparison means is that the accumulator charge is controlled, as soon as the supply voltage becomes less than the minimum threshold.
According to another feature of the transponder according to the present invention, the minimum threshold corresponds to the minimum level of supply voltage necessary to assure all the functions of the transponder, which has the advantage of supplying the transponder with sufficient power to assure all the functions of the transponder.
Another advantage of the comparison of the first supply voltage to such a minimum threshold is that it prevents the accumulator being completely run down during the operation thereof, which avoids short-circuiting the processing means.
According to another feature of the transponder according to the present invention, the first comparison means can compare the first supply voltage to a maximum threshold which represents the upper accumulator charge limit, which has the advantage of avoiding overloading the accumulator.
According to another feature of the transponder according to the present invention, the processing means are arranged for receiving the electric power from the storage means when the accumulator is charging, and from the accumulator once it is charged. One advantage of such an arrangement of the processing means is that centralised control of the electric power supply of the transponder components can be achieved.